This application claims rights of priority under 35 U.S.C. xc2xa7 119 based on Russian Patent Application No. 97112675, filed Jul. 5, 1997.
1. Field of the Invention
This invention is related to the field of organic synthesis, and in particular, to the methods for preparing hydroxylated aromatic compounds (e.g., phenol and its derivatives), by selective oxidation of aromatic compounds (e.g., benzene and its derivatives), with gaseous mixtures comprising nitrous oxide in the presence of heterogeneous catalysts. Commercial zeolites or zeolite-containing catalysts modified by special treatments described herein are used as heterogeneous catalysts.
2. Description of the Prior Art
Various processes are known in the art for preparing phenol and its derivatives, such as diphenols, cheorophenols, fluorophenols, alkylphenols and the like. Known processes include direct oxidation of aromatic hydrocarbons or their derivatives with O2, N2O or other gaseous oxidants in the presence of oxide catalysts such as those referenced in U.S. Pat. No. 5,110,995. However, the majority of the known oxide catalysts for the direct oxidation of benzene to phenol in the presence of molecular oxygen, do not provide high selectivity and yield of the target product. The most successful example of such a catalyst is prepared from phosphates of various metals. In particular, ZnPO4 has been used as a catalyst for benzene oxidation into phenol in the presence of alcohols.
At temperatures of 550-600xc2x0 C., the ZnPO4 catalyst produced a phenol yield of about 25%. However, the selectivity of ZnPO4 was poor (60%) [Japan Patent No. 56-77234 and 56-87527, 1981]. Furthermore, phosphate catalysts are disadvantageous for benzene oxidation because they consume substantial quantities of alcohols.
Vanadium-, molybdenum-, or tungsten-based oxide catalyst systems for direct benzene oxidation with nitrous oxide (N2O) at 500-600 xc2x0 C. are known [Iwamoto et al., J. Phys. Chem., 1983, v. 87, no. 6, p. 903]. The maximum phenol yield for such catalysts in the presence of an excess of steam is about 7-8%, with a selectivity of 70-72%. The main drawbacks of these catalysts are their low selectivity and yield of phenol, the required high temperatures for the reaction, and the requirement to add steam.
Zeolite catalysts are also available for the selective oxidation of benzene and its derivatives using N2O as an oxidant (E. Suzuki, K. Nakashiro, Y. Ono, Chem. Lett., 1988, no. 6, p. 953-1 M. Gubelmann et al., Eur. Pat., 341,165, 1989-1 M. Gubelmann et al., U.S. Pat. No. 5,001,280, 1990). Specifically, high-silica ZSM-5 type pentasil zeolites are used as catalysts for oxidation of benzene, chlorobenzene, and fluorobenzene into corresponding phenols. The oxidation of benzene with nitrous oxide on HZSM-5 zeolite at 400xc2x0 C. leads to the formation of phenol with a yield up to 16%, and a selectivity close to 98-99%. The disadvantage of these catalysts is that they have low conversion rates, low yields of phenol and low selectivity at high reaction temperatures.
The zeolites of the pentasil type (e.g., ZSM-5, ZSM-11, ZSM-12, ZSM-23), mordenite, zeolite Beta and EU-1, which are all modified with small iron additives during their synthesis, are known systems for performing this catalytic reaction. For example, in U.S. Pat. Nos. 5,672,777 and 5,110,995, experimental results are presented for benzene oxidation with nitrous oxide at 275-450xc2x0 C. The contact time was 2-4 sec, the liquid space velocity of benzene was 0.4 hxe2x88x921, and the molar benzene : N2O ratio was 1:4. The phenol yield typically reached 20-30%, and the selectivity was 90-97%. The disadvantages of these catalysts include the necessity to introduce iron ions into the zeolite and to control the oxidation state of iron ions, the low liquid space velocity value of benzene, the significant contact time necessary to obtain acceptable, but not impressive yields of the final product, and the low selectivity at elevated temperatures (xcx9c450 C).
An HZSM-5 type catalyst that is dehydroxylated at a high temperature is also known in the art (V.L. Zholobenko, Mend. Commun., 1993, p. 28). This high temperature dehydroxylation pretreatment was found to increase the phenol yield from xcx9c12 to xcx9c20-25 wt. % at the N2O:benzene ratio of 4:1. However, this catalyst also produced a low yield of phenol. In the process described above, the high-temperature dehydroxylation was performed in one stage with no control of the nature of the zeolite active sites. Therefore, in this process, the formation of both framework and extra framework active sites was quite possible. The significant disadvantage of all these methods is that they require a large excess of N2O over the hydrocarbon (e.g., benzene) to provide more complete conversion of the hydrocarbon to the desired oxidation products.
Another method of benzene oxidation was proposed in the patent by Panov G. I. et al. (PCT W095/27691). In this method, an excess of benzene over N2O was used (up to 9:1), and the selectivity of N2O conversion into phenol was improved. However, in this case, the catalyst contained iron as an active component Such catalysts are problematic because the oxidation state of the iron introduced into such a catalyst must be controlled. Also, the yield of phenol barely exceeded 20 wt. %, although the benzene liquid hourly space velocity (hereinafter xe2x80x9cLHSVxe2x80x9d) was increased as compared to the previous systems to about 2-2.5 hxe2x88x921.
In another known method, phenol is produced by oxidative hydroxylation of benzene and its derivatives with nitrous oxide at 225-450xc2x0 C. in the presence of an iron-containing zeolite catalyst. This zeolite catalyst is pretreated at 350-950xc2x0 C. in steam containing 0.1-100 mol. % H2O (Kharitonov A. S., et al., U.S. Pat. No. 5,672,777, 1997xe2x80x94Russian Patent No. 2074164, C07C 37/60, June 1997-1 Application No. 94013071/04, C07C 37/60, 27.12.1995). However, treatment of the zeolite catalyst using this method does not cause a substantial increase in the activity. Another drawback of this method is the low stability of the resultant catalyst, which deactivates during the oxidation process due to the formation of tar-like side-products. Another disadvantage of all the methods described above is the low partial pressures of benzene in the vapor mixturexe2x80x94the benzene content was 5 mol. % and the partial pressure of benzene was about 40 torr.
Thus, an object of the present invention is to develop a method of preparing hydroxylated aromatic compounds (e.g., phenol and derivatives) by selective oxidation of aromatic compounds (e.g., benzene and its derivatives). Specifically, it is an object of the invention to use N2O as a mild oxidant in the presence of an appropriate catalyst that enhances productivity of the oxidation process by increasing the yield of hydroxylated aromatics and selectivity for the target product. It is a further object of the invention to simultaneously minimize the consumption of N2O by decreasing the oxidant-to-hydrocarbon ratio in the feed, and increasing the efficiency of N2O conversion to the desired oxidation products. It is also an object of the invention to avoid producing side products.
The objects of the invention are accomplished by a method of preparing hydroxylated aromatic compounds (e.g., phenol or its derivatives) by oxidation of aromatic compounds (e.g., benzene and derivatives) with nitrous oxide. The method of the present invention significantly increases the process efficiency due to the increase in the activity and selectivity of the catalyst, and the increase in the yield of the target products (i.e., hydroxylated aromatic compounds). In order to achieve these results, the aromatic compounds are oxidized using nitrous oxide at 225-500xc2x0 C. in the presence of a zeolite catalyst. The zeolite catalyst according to the invention is modified with strong Lewis acid-base sites of a specific nature. These sites can be introduced into the zeolite catalyst by performing a special high-temperature pretreatment. This preliminary thermal activation of the H-form of zeolite is carried out in two steps. In the first step, the catalyst is heated at 350-450xc2x0 C. for 4-6 h in an inert gas (nitrogen or helium) or air stream. In the second step, the catalyst is calcined at 450-1000xc2x0 C. for 1-3 h in a continuous flow of an inert gas or air followed by cooling the zeolite catalyst to the reaction temperature (typically 300-450xc2x0 C.). In a preferred version of the invention, the hydroxylated aromatic compounds are phenol and its derivatives, and the aromatic compounds are benzene and its derivatives.
Applicants do not wish to be bound by any particular theory of operation of the invention. However, Applicants offer the following explanation of how the temperature treatment affects the catalyst. The purpose of the two-step high-temperature treatment is related to the generation of a specific type of Lewis acid-base pair centers, preferably framework Lewis acid-base sites. This is achieved by separating the stage of removal of adsorbed water and/or ammonium ions (which are introduced via ion exchange at the stage of the preparation of an Hxe2x80x94 or NH4-forms of zeolites), from the stage of removing structural (bridging) OH groups intrinsic to the H-zeolite framework. For this purpose, the thermal treatment is carried out in two steps. In the first step, the zeolite is calcined at a temperature up to 350-450xc2x0 C. (a conventional pretreatment). In this first step, adsorbed water and exchanged ammonium ions are intensively removed. In the second step, the zeolite is calcined at temperatures ranging from 450 to 950xc2x0 C., depending on the zeolite composition. In this second step, structural (acidic) OH groups of zeolites are removed. This second step can solve two problems: (1) removing acidic OH groups that are the active sites for side reactions leading to the formation of tar-like products; and (2) creating new (aprotic) rather strong Lewis acid-base pairs, preferably related to the framework of the zeolite, that are capable of activating N2O molecules to cause evolution of molecular nitrogen and formation of atomic oxygen species adsorbed on strong Lewis acid sites. The atomic oxygen acts as a mild oxidizing agent in the reaction of selective oxidation of aromatic compounds to corresponding hydroxylated aromatic compounds. The strong Lewis acid-base centers as precursors of the active oxidizing centers (atomic oxygen) can be detected by IR spectroscopy using adsorbed probe-molecules, such as CO, H2, CH4, etc.
According to the present invention, the starting materials for the preparation of the zeolite catalysts are the commercial forms of zeolites, such as:
(1) high-silica pentasil-type zeolites like ZSM-5, ZSM-11 etc., prepared, for instance, as described in U.S. Pat. No. 3,702,886, which is hereby incorporated by reference;
(2) zeolite H-mordenite; or
(3) isomorphously substituted pentasils like ferrisilicate, gallosilicate etc.
Preferably, a commercial ZSM type zeolite (ZSMe-5, ZSM-11, ZSM-12, ZSM-23 etc.) with Si/Al or Si/Me ratios (where Me=Ga, Fe) greater than 20 is used in the present invention. In more preferred versions of the invention, the Si/Al or Si/Me ratio ranges from 40 to 100.
According to the present invention, the commercial zeolite is acidified by addition thereto of an inorganic or organic acid. In a preferred embodiment of the invention, the zeolite is acidified by soaking it with from 10 ml to 100 ml of acid per gram of a zeolite, wherein the acid has a normality of from 0.1 N to 2 N. The acid soaking may be done in a single step, or more preferably, in several steps.
Acid forms of zeolite may be also prepared by exchanging of a commercial zeolite with an aqueous solution of an ammonium salt (e.g., a nitrate or chloride salt). For example, a Na-form of ZSM-type zeolite is treated with a 0.1-2 N solution of an appropriate ammonium salt The ion exchange degree of sodium for ammonium or protons is varied from 30 to 100%, and more preferably from 50 to 95%.
Zeolites can be used as catalysts in the pure form or in a combination with an appropriate binder. In a preferred embodiment of the invention, amorphous silica with a specific surface area ranging from 100 to 600 m2/g, or alumina with a specific surface area ranging from 100 to 400 m2/g, or a mixture thereof, are used as binders. The content of the binder in the catalyst ranged from 5 to 50 wt %, and more preferably from 20 to 30 wt %.
Nitrous oxide may be employed alone, or in admixture with an inert gas such as nitrogen or helium, or in admixture with air.
Aromatic hydrocarbons, such as benzene, toluene, ethylbenzene, cumene, xylenes and the like, the halogenated aromatic compounds such as chlorobenzene, fluorobenzene, difluorobenzenes and the like, phenol, styrene or a mixture thereof are typically used as substrates for selective oxidation with nitrous oxide. It is also possible to selectively further oxidize an aromatic compound such as phenol, using the process described herein. For purposes of this specification, these substrate materials will be generally referred to as xe2x80x9caromatic compounds.xe2x80x9d
In the process described herein, the substrate is typically introduced in a mixture with nitrous oxide in a molar ratio of nitrous oxide to substrate ranging from 1:7 to 5:1, and more preferably, from 1:2 to 4:1. The LHSV of the substrate ranged from 0.2 to 5 hxe2x88x921, more preferably from 0.5 to 2 hxe2x88x921. The reaction is preferably carried out at a temperature from 300 to 500xc2x0 C., and more preferably from 350 to 450xc2x0 C. The contact time of the reaction mixture with a catalyst ranges from 0.5 to 8 sec, and more preferably from 1 to 4 s.
The gases evolved from the reactor may comprise a mixture of phenol and dihydroxybenzenes and are condensed and separated by any technique known to this art (GC, LC, MS or a combination thereof).
The catalyst can be easily and reversibly regenerated by calcination at 400-600xc2x0 C. in a flow of air, oxygen, and nitrous oxide, or mixtures thereof with an inert gas. The regeneration is carried out for 1-3 h.
In order to further illustrate the present invention and the advantages thereof, the following specific examples are given, it being understood that same are intended only as illustrative and in no way limitative.